An apparatus for the two-sided coating of a web of paper or cardboard can comprise two juxtaposed pressing rollers defining a pressing gap or nip between them through which the web can pass. An applicator and metering system can be provided for each of these rollers, upstream of the nip with respect to the direction of rotation of the roller to apply to the roller a layer of a material, usually a liquid or other flexible substance, transferred to the web at the nip.
Such apparatus can be employed for the two-sided coating of glue or sizing (starch, carboxymethylcellulose-CMC, synthetic size or glue) or pigment dispersions onto paper or cardboard webs.
The purpose of the applicator and metering or dosing system assigned to each roller is the application to the respective roller of a film of the coating material in a metered amount.
An apparatus of this type is described in the German Utility Model 84 14 413.
In this system, the applicator and metering unit includes a nozzle chamber open toward the respective pressing roller, and at a downstream side of this chamber, a metering element which is pressed against the pressing roller and which closes the nozzle chamber.
The metering elements which can be used can include a doctor bar formed with peripheral grooves or flutes and fabricated, for example, by adjacent turns of wire on a support. The amount of the flowable substance which is applied to the pressing roller is then determined by the groove cross-section where the doctor bar is applied forceably against the surface of the pressing roller.
Other known metering elements include doctor bars with smooth surfaces, shaver blades or shaver strips and, in general, doctor blades.
In these systems, the coating weight applied to the surface of the pressing roller, i.e. the weight of material per unit area, can be varied by varying the pressing force with which the doctor bar or doctor blade is urged toward the pressing roller.
It is known to measure the quantity of the coating substance applied to a web continuously and directly upon the web. For this purpose, radioactively-emitting elements may be provided and the absorption of the radioactive radiation through the web can indicate the coating weight (quantity in weight units per unit area) and/or the moisture of the coating applied to the web which, in turn, can be correlated with the quantity of coating applied per unit area and, for a given and known substance, the thickness thereof or the amount of the substance coated onto or penetrating into the web.
For adjustment and/or monitoring of the coating weight, separately for each side, complex measuring devices must be provided and juxtaposed with each side of the web directly and, as experience has shown, can provide with any accuracy only the total quantity of coating material applied by the two-sided coating without any significant possibility of accurately indicating the quantity applied to each side.
Mention may also be made of U.S. Pat. No. 4,957,770 which provides a sensor and a method for determining the basis weight, i.e. the weight per unit area, of coating material on a substrate.
The infrared sensor of this apparatus determines the amount of a coating material on a substrate using measurements of infrared radiation reflected from the substrate or the transmission of infrared radiation through the substrate. This system employs at least two separate weavelength regions of the infrared.
More particularly, the infrared coating sensor includes a source of infrared radiation, a beam of which is transmitted toward the moving sheet. When the beam reaches the sheet, it first passes through the coating material and then into the base paper sheet. A portion of the energy is transmitted through the sheet. Some of the infrared energy, after entering the base sheet, is reflected back in the general direction of the source. It is recognized in this reference that infrared radiation is preferentially absorbed by the coating and/or the base sheet itself.
Notwithstanding the ability to utilize infrared measurements to determine the coating on the sheet, the sensor and the use of it in this patent do not assure extremely precise applications of the coating substances to both sides of a web of paper or paper board.